Vaccinia virus and more recently other poxviruses have been used for the insertion and expression of foreign genes. The basic technique of inserting foreign genes into live infectious poxvirus involves recombination between pox DNA sequences flanking a foreign genetic element in a donor plasmid and homologous sequences present in the rescuing poxvirus (17).
Specifically, the recombinant poxviruses are constructed in two steps known in the art and analogous to the methods for creating synthetic recombinants of the vaccinia virus described in U.S. Pat. No. 4,603,112, the disclosure of which patent is incorporated herein by reference.
First, the DNA gene sequence to be inserted into the virus, particularly an open reading frame from a non-pox source, is placed into an E. coli plasmid construct into which DNA homologous to a section of DNA of the poxvirus has been inserted. Separately, the DNA gene sequence to be inserted is ligated to a promoter. The promoter-gene linkage is positioned in the plasmid construct so that the promoter-gene linkage is flanked on both ends by DNA homologous to a DNA sequence flanking a region of pox DNA containing a nonessential locus. The resulting plasmid construct is then amplified by growth within E. coli bacteria (20) and isolated (21,22).
Second, the isolated plasmid containing the DNA gene sequence to be inserted is transfected into a cell culture, e.g. chick embryo fibroblasts, along with the poxvirus. Recombination between homologous pox DNA in the plasmid and the viral genome respectively gives a poxvirus modified by the presence, in a nonessential region of its genome, of foreign DNA sequences. The term "foreign" DNA designates exogenous DNA, particularly DNA from a non-pox source, that codes for gene products not ordinarily produced by the genome into which the exogenous DNA is placed.
Genetic recombination is in general the exchange of homologous sections of DNA between two strands of DNA. In certain viruses RNA may replace DNA. Homologous sections of nucleic acid are sections of nucleic acid (DNA or RNA) which have the same sequence of nucleotide bases.
Genetic recombination may take place naturally during the replication or manufacture of new viral genomes within the infected host cell. Thus, genetic recombination between viral genes may occur during the viral replication cycle that takes place in a host cell which is co-infected with two or more different viruses or other genetic constructs. A section of DNA from a first genome is used interchangeably in constructing the section of the genome of a second co-infecting virus in which the DNA is homologous with that of the first viral genome.
However, recombination can also take place between sections of DNA in different genomes that are not perfectly homologous. If one such section is from a first genome homologous with a section of another genome except for the presence within the first section of, for example, a genetic marker or a gene coding for an antigenic determinant inserted into a portion of the homologous DNA, recombination can still take place and the products of that recombination are then detectable by the presence of that genetic marker or gene in the recombinant viral genome.
Successful expression of the inserted DNA genetic sequence by the modified infectious virus requires two conditions. First, the insertion must be into a nonessential region of the virus in order that the modified virus remain viable. The second condition for expression of inserted DNA is the presence of a promoter in the proper relationship to the inserted DNA. The promoter must be placed so that it is located upstream from the DNA sequence to be expressed.
The technology of generating vaccinia virus recombinants has recently been extended to other members of the poxvirus family which have a more restricted host range. The avipoxvirus, fowlpox, has been engineered as a recombinant virus expressing the rabies G gene (23,24). This recombinant virus is also described in PCT Publication No. WO89/03429. On inoculation of the recombinant into a number of non-avian species an immune response to rabies is elicited which in mice, cats and dogs is protective against a lethal rabies challenge.
It is well established that one of the antiviral mechanisms induced by interferon (Ifn) is the inhibition of the initiation of protein synthesis due to the phosphorylation of eIF-2alpha by the P1 kinase (1,2). Vaccinia virus (VV) has been shown to be relatively resistant to Ifn (3,4) and is capable of rescuing Ifn-sensitive viruses from the effects of Ifn (5,6), by somehow reducing the level of eIF-2alpha phosphorylation.
VV-based vaccines are useful immunizing agents (14). Recombinant poxvirus vaccine candidates, particularly VV vaccine candidates, having enhanced sensitivity to interferon, would have an increased level of safety. An Ifn-sensitive phenotype would provide a means for drug intervention in the event that vaccination leads to vaccinial complications.
It can thus be appreciated that provision of a recombinant poxvirus, particularly recombinant vaccinia virus, having enhanced sensitivity to interferon, would be a highly desirable advance over the current state of technology.